Study subjects will be randomized into one of two groups, an intervention group receiving oral probiotic supplements, and a control group not receiving probiotics. Accrual of subjects will occur in a 2:1 ratio of probiotic ingestors:controls. All subjects will participate in a continuous 28 week study. In the intervention/probiotics group, this will comprise a 4-week baseline observation period, three successive probiotic ingestion periods of 6 weeks each, and a post-ingestion off-probiotic observation period of 6 weeks. During the first 6 weeks of probiotic ingestion, the subjects will take one probiotic tablet daily. This will be increased to one tablet twice daily during the second 6-week period and finally to one tablet three times daily during the third consecutive 6-week period. Subjects will have blood samples drawn every 2 weeks throughout the 28-week study period for quantitation of serum isoagglutinin titers. The outcome measure is the percent of probiotic ingestors versus controls who experience a four-fold or greater rise in isoagglutinin titer in association with a final titer of at least 128. To date, 36 subjects have accrued to the study, all of whom are still within the first 20 weeks of probiotic ingestion (or controls). Plasma samples drawn every two weeks were cryopreserved for assay of isoagglutinin titers in a batch format at a later date.